Many liquefied natural gas (LNG) plants have seasonal fluctuations in production capacity, with higher production potential during the colder months of the year and lower production potential during the warmer months of the year. One reason for the reduced production during warmer months is that since ambient temperature is higher, the density of the air fed to gas turbines used in the liquefaction process is reduced, and thus turbine efficiency and power output are reduced. Another reason for the reduced production is that since ambient temperature is higher, the vapor pressures of all refrigerants used increase so that refrigerant vapors must be compressed at higher pressure, imposing a greater horsepower load on the refrigerant circuit. The opposite effects are observed during the colder months. Another process aspect which can significantly impact the designed plant capacity is the varying richness of feed gas. Typically more power is required to separate natural gas liquids (NGL) from a rich gas stream. All of the facilities involved with producing LNG therefore need to be sized and designed to accommodate all conditions between the minimum production (e.g., summer) and maximum production (e.g., winter) operating cases. Such facilities include upstream facilities, e.g., wells, inlet separators, dehydration units, gas processing facilities and natural gas liquids removal facilities, liquefaction facilities, e.g., main cryogenic heat exchangers, refrigeration loops, and supporting utilities, e.g., gas turbine generators, and downstream facilities, e.g., nitrogen rejection units, end flash gas handling and LNG storage units. An ongoing challenge is to develop new systems and processes to enhance LNG production year-round, in a way that minimizes capital investment, operating costs, added equipment footprint, and significant modifications to the main LNG plant.
Small-scale liquefaction units, also referred to as “packaged” liquefaction units or small-scale LNG plants, having a capacity of less than 2 MTPA (million tons per annum) have been developed. An example is the PRICO® single-mixed refrigerant process available from Black & Veatch (Overland Park, Kans.), disclosed in PCT Publication No. WO 2009/151418. It would be desirable to apply such small-scale liquefaction units to produce incremental LNG in tandem with a full-scale LNG plant having a capacity of at least 4 MTPA. However, a pretreated gas stream fed from the upstream gas processing facilities of the full-scale LNG plant to the small-scale liquefaction unit requires significant refrigeration in order to be liquefied. Furthermore, a pretreated gas stream fed from the upstream gas processing facilities of the full-scale LNG plant to the small-scale liquefaction unit may require separate natural gas liquids removal facilities.
It would be desirable to apply a small-scale liquefaction unit to enhance the LNG production of a full-scale LNG plant in a way that avoids the aforementioned disadvantages. There is a large economic incentive for even small capacity improvements.